This invention relates to an antenna arrangement for a vehicle window.
It has increasingly become a requirement for equipment within vehicles to be able to transmit and receive in the UHF band, for example, for use in connection with cellular telephone, satellite navigation systems or for other uses.
Commonly, to provide for UHF reception, a vehicle is provided with a whip antenna which is in length a multiple of one quarter of the wavelength of the signal to be received. Alternatively, a single vertical stub antenna may be mounted on a window, usually the rear window of the vehicle.
A problem with the arrangements described above is that antennas are subject to multipath reception which leads to fast fading of the signal received, which, in the case of an audio signal, causes a general xe2x80x9cbreak-upxe2x80x9d in reception. Multipath reception is a consequence of signals being received by the antenna both directly from a transmission source and after reflection from different surfaces. Commonly, such surfaces include walls of building in a built-up urban environment.
It is an aim of the present invention to provide an antenna arrangement in which the effect of fast fading upon the quality of the received signal can be reduced.
According to a first aspect of the present invention therefore there is provided An antenna comprising receiving elements and a base element the base element acting as a ground plane or as a counterpoise resonant element, each of which elements are formed of electrically-conductive material, characterised in that the receiving elements are formed as conductive strips, angled to one another and to the ground plane, and in that signals from the antenna may be derived from two outputs corresponding respectively to the sum and the difference of outputs of the receiving elements, whereby the antenna is operable in orthogonal modes when placed on a window pane.
The modes being suitable for providing diversity reception at high frequencies, output signals being sourced selectively from whichever mode provides the stronger output signal. This arrangement at least mitigates, the abovementioned fast fading problems associated with previously used arrangements.
Signals may be fed to and from the modes by means of a hybrid element, which, in operation, permits the modes to be combined without affecting the operation of the antenna itself. The hybrid element may comprise a wound transformer or a conductive ring structure.
The ground plane may be constituted by an edge of the window or alternatively may comprise a conductive ground strip printed on the window. More preferably, a tuned conductive element is provided (to constitute a counterpoise resonant element) instead of the ground plane.
The antenna may have two elements symmetrically inclined about a median plane which extends normal to an centrally of the ground plane or the resonant element, as the case may be.
In a preferred form, each element comprises a loop of conductive material. A first end of each loop may be connected to the base element, second ends of the loops being interconnected at a common point. In such embodiments, signals may be fed to and/or from the antenna at the common point.
In an alternative mode of operation of an antenna embodying the invention, the two modes are operable separately or in combination through a hybrid element, signals being connected with the antenna through a circuit operative to combine the two modes in appropriate phase and amplitude to produce, in effect, single antenna of an optimised performance. As will be discussed below, this mode of operation is particularly advantageous when the antenna is to be used to transmit signals.
In some circumstances, the two modes may each exhibit resonance two or more at substantially different frequencies, so enabling reception and/or transmission of signals in different frequency bands.
An antenna embodying the invention may be used in combination with a switching circuit whereby the antenna is operable as an adaptive antenna system having directional characteristics variable in real time under automatic control.
Most advantageously, an antenna embodying the invention is formed as a pattern of conductors printed onto a glass pane. These conductors may be formed at very low cost as part of the process whereby a heater is formed on the pane. In such embodiments, a portion of the pattern may constitutes a hybrid element, and a further portion of the pattern may constitute an impedance matching element.
An antenna embodying the invent ion may be provided as part of an antenna system in combination with another antenna.
Although it is not possible to use a diversity antenna system for transmission, the antenna of the present invention is advantageous when used as a transmission antenna. A major problem associated with transmission of high-frequency signals from vehicles arises from the interaction between the antenna and the conductive vehicle body. It has been found in practice that the nature of the interaction varies substantially from one model of vehicle to another, with the consequence that it has not hitherto been possible to produce a generic transmission antenna optimised for use in a wide range of vehicles.
In a second of its aspects the invention provides a transmission antenna comprising an antenna according to the first aspect of the invention, and a combining and tuning circuit, in which the combining and tuning circuit adjusts the directional and bandwidth characteristics of the antenna to the vehicle with which it is intended for use.
Thus, a common antenna can be used on a wide range of vehicles, it being necessary to tailor only the combining and tuning circuit to the characteristics of the vehicle.
As will be appreciated by those skilled in the art, a transmission antenna which has two separably operable orthogonal modes can, with suitable combining and tuning, exhibit an extremely diverse range of directional, polarisation and other characteristics. Such a circuit typically imposes a phase shift or a delay in the signal fed to one of the modes with respect to the other, and which after combination effects a relative difference in the magnitudes of the signals.